Strava has a suffer score, it basically works out how much of a run made you actually work. Probably some clever algorithm behind it. However on running back from dropping off my car at the garage (3km) I've worked out an easy alternative for non Strava users.
If you do 1k-it is knackering, 2k - kknackering, 3k kkknackering etc
If you work in miles then mildly tiring? Suggestions please.